The present invention relates generally to electronic circuits adapted to cooperate with transformer relays.
Electro-magnetic devices have been used to control high-voltage, high current electrical loads with remotely located low voltage switches. Generically these devices are called transformer relays. For purposes of this application, a transformer relay includes unitary electro-magnetic devices and a combination of a transformer and a relay coupled to perform a similar function.
One transformer relay is disclosed in U.S. Pat. No. 3,461,354, Bollmeier, MAGNETIC REMOTE CONTROL SWITCH, assigned to the assignee of the present application. Another transformer relay is disclosed in U.S. Pat. No. 4,321,652, Baker, et al, LOW VOLTAGE TRANSFORMER RELAY, issued Mar. 23, 1982 and assigned to the assignee of the present application, which provides an improved version over the Bollmeier transformer relay. Both the Bollmeier patent and the Baker application are incorporated herein by reference.
The transformer relays as described in Bollmeier and Baker are controlled by a directional flow of current in a secondary winding of the transformer relay above a predetermined switching threshold. A typical use of these transformer relays would be to connect them to one or more bidirectional switches. For purposes of this application, a bidirectional switch is a switch which is controlled by an operator to selectively allow only unidirectional flow of current between its terminals.
One principal advantage of a transformer relay is the ability to control one transformer relay from a plurality of locations and also to control a plurality of transformer relays from one switch location. An example would be to control a light, or a group of lights, in an office building from several different locations as well as from a single, central location.
U.S. Pat. No. 4,338,649, Mosier, SYSTEM FOR REMOTELY CONTROLLING A LOAD, issued July 6, 1982 and assigned to the assignee of the present application, discloses a system utilizing electronics to couple a plurality of bidirectional switches with a transformer relay. U.S. Pat. No. 4,384,314, Doty, et al, CONTROL SYSTEM FOR A PLURAL TRANSFORMER RELAY, issued May 17, 1983 and assigned to the assignee of the present application, discloses a control system for allowing a bidirectional switch to control a plurality of transformer relays. Both of these applications disclose interface circuits or control systems for utilizing a plurality of transformer relays and/or a plurality of bidirectional switches.
A problem, however, exists in the use of transformer relays with multiple bidirectional switches. In the case where a plurality of bidirectional switches are coupled directly to the secondary winding of a transformer relay, simultaneous activation, in opposite directions, of at least two bidirectional switches results in an effective short across the secondary winding of the transformer relay. This may cause the transformer relay to switch following the waveform of the excitation power. For a 60 Hertz power system, this may cause the transformer relay to switch 120 times per second, once for each half cycle of the power waveform. This relatively rapid switching may produce a great deal of heat within the load switching contacts and, if continued, could result in permanent damage to the transformer relay. The interface circuits and control systems disclosed in the Mosier and Doty applications, do not alleviate this problem, the interface circuits and control systems being designed for different purposes.